Wild European Eels Research Articles

Gediz Deltası (Ege Denizi)’ndan avlanan Avrupa yılanbalığında (Anguilla anguilla) yüzme kesesi paraziti Anguillicoloides crassus enfeksiyonu

The wild European eels (776 fish, mean total length of 547±91 mm, mean weight 340±198 g) were obtained monthly between October 2015-September 2016 in Gediz Delta (Izmir Bay, Aegean Sea). A total of 263 nematode parasites (Anguillicoloides crassus) were isolated from the swim bladders of 52 parasitized fish. Parasitological indices were found relatively low. The prevalence was 6.7% and the mean intensity was 5.05. The abundance of A. crassus was also calculated as 0.33 in all sampled eels.

Impact of environmental micropollutants and diet composition on the gut microbiota of wild european eels (Anguilla anguilla)

In fish, the gut microbiome plays a crucial role in homeostasis and health and is affected by several organic and inorganic environmental contaminants. Amphidromous fish are sentinel species, particularly exposed to these stressors. We used whole metagenome sequencing to characterize the gut microbiome of wild European eels (Anguilla anguilla) at a juvenile stage captured from three sites with contrasted pollution levels in term of heavy metals and persistent organic pollutants. The objectives were to identify what parameters could alter the gut microbiome of this catadromous fish and to explore the potential use of microbiota as bioindicators of environment quality.We identified a total of 1079 microbial genera. Overall, gut microbiome was dominated by Proteobacteria, Firmicutes and Actinobacteria. Alpha and beta diversity were different amongst sites and could be explained by a reduced number of environmental and biological factors, specifically the relative abundance of fish preys in eels’ diet, PCB101, γHCH (lindane), transnonachlor and arsenic. Furthermore, we identified a series of indicator taxa with differential abundance between the three sites. Changes in the microbial communities in the gut caused by environmental pollutants were previously undocumented in European eels. Our results indicate that microbiota might represent another route by which pollutants affect the health of these aquatic sentinel organisms.

Virus infections of the European Eel in North Rhine Westphalian rivers

Viral infections have been suggested to play a role in the decline of the panmictic population of the European eel (Anguilla anguilla). However, despite the importance of knowledge about pathogenic eel viruses, little is known about their spread in the wild European eel population and only a few eel pathogenic viruses have been described so far. In this study, we aimed to investigate the health status of the A.anguilla stock in North Rhine Westphalia (NRW) State of Germany. For this purpose, we examined tissue samples of 16 elvers, 100 yellow eels and 6silver eels, sampled from the rivers Rhine, Lippe and Ems. Virus detection was performed via a combination of cell culture and PCR. Next to the detection of frequently encountered pathogenic eel viruses (anguillid herpesvirus 1 and eel virus European X (EVEX)), we isolated the eel picornavirus 1 (EPV-1) from tissue of yellow eels and elvers and demonstrate the distribution of EPV-1 in wild eel population in NRW.

Can inorganic elements affect herpesvirus infections in European eels?

In combination, pollution and pathogens represent a serious threat to the health of European eels that has been increasingly recognized. Thus, the impact of contaminants, cadmium, lead, mercury, and selenium, on anguillid herpesvirus 1 infection in wild European eels has been evaluated. Despite the small sample size, results indicate that selenium and mercury concentrations may compromise the European eel immune system as herpesvirus infection was more prevalent in specimens with higher Hg and Se hepatic concentrations.

First isolation of Flavobacterium psychrophilum associated with reports of moribund wild European eel (Anguilla anguilla) in Scotland.

In late April 2015, the River Dee Trust informed Marine Scotland Science, Fish Health Inspectorate (FHI), that there had been observations of dead and moribund European eels on the River Dee. Later in May, the Spey Fishery Board also reported a number of moribund European eels in a rotary screw smolt trap on the River Spey. In total, 10 cases involving moribund eels were investigated in 2015 and one case in 2016. In addition, a health screen was conducted to investigate the potential presence of Flavobacterium psychrophilum in healthy eels and Atlantic salmon from the River Dee in 2015. Externally, the diseased eels demonstrated white patches in different locations of the body. In all cases, F.psychrophilum was detected by bacterial isolation and/or molecular methods. Three isolates were further characterized by whole-genome sequencing (WGS) as belonging to sequence type 15 (ST15). Histological examination of diseased European eels revealed lesions at the level of the integument. The pathogen screen for F.psychrophilum in wild healthy fish tested negative by PCR. Further investigation is required to understand the pathogenicity of this bacterium on the health of eels and the potential impact on the wild salmonid population.

Authentication of farmed and wild european eel (Anguilla anguilla) by fatty acid profile and carbon and nitrogen isotopic analyses

We determined fatty acid profile and protein carbon and nitrogen isotopic composition in muscle of wild and farmed eels, in order to discriminate the geographical origin of fish and their dietary habits. The samples were divided into groups according to the farm origin (Italy, Denmark and Netherland), the market size (capitoni and buratelli) and the farming system (pond, RAS, lagoon and wild). Biometric measurements, proximate composition analysis (moisture, ash, lipid and protein content), fatty acids profile and carbon and nitrogen stable isotopes measurements were performed. Statistical analysis was fulfilled to investigate the homogeneity of variance and the means distribution, including Principal Component Analysis and Partial Least Squares Discriminant Analysis. The results suggested that the different farming conditions represented the main factor affecting fish muscle composition. In this scenario, fatty acid profile and δ13C and δ15N values of the defatted muscle represented useful tools to distinguish between eels reared under different feeding regimes (wild vs captive-bred). No differences were found between eels reared in different farms.

Seasonal recovery of Edwardsiella piscicida from wild European eels and natural waters: Isolation methods, virulence and reservoirs.

A total of 127 wild eels caught in the L'Albufera Lake (Spain) and 24 samples of lagoon freshwater were analysed for 1-year period. Edwardsiella strains were isolated from liver/kidney on TSA-1 plates in 31.9% of total diseased specimens, and the edwardsiellosis prevalence in the fishery was of 11.8%. The use of double-strength Salmonella-Shigella (DSSS) broth and SS agar yielded Edwardsiella isolation from intestine in 100% of those edwardsiellosis-diseased eels, but also in 40.4% of other sick fish with vibriosis or aeromonosis and in 28.8% of healthy eels, as well as from freshwater in 8.3% of samples. Pure cultures were isolated on SS agar from the former, but motile Aeromonas, Plesiomonas shigelloides and Hafnia alvei were recovered along with Edwardsiella in the other samples. Edwardsiella isolates identification at species level revealed that E.piscicida was distributed between wild eels and freshwater but E.tarda only did in freshwater. All E.piscicida strains were virulent for eels (LD <1.0×106 CFU/fish) but that of E.tarda was not. This is the first report of E.piscicida in wild eel intestines and natural freshwater, highlighting its role as potential reservoirs for the bacterium. A seasonal recovery was found for E.piscicida at water temperature above 20°C.

Organophosphorus flame retardants in the European eel in Flanders, Belgium: Occurrence, fate and human health risk

The present study investigated the levels, profiles and human health risk of organophosphorus flame retardants and plasticizers (PFRs) in wild European eels (Anguilla anguilla) from freshwater bodies in the highly populated and industrial Flanders region (Belgium). Yellow eels (n=170) were collected at 26 locations between 2000 and 2009 and for each site, muscle samples of 3–10 eels were pooled and analyzed (n=26). Muscle lipid percentages varied widely between 2.4% and 21%, with a median value of 10%. PFRs were detected in all pooled samples in the order of tris-2-chloroisopropyl phosphate (TCIPP)>triphenyl phosphate (TPHP)>2-ethylhexyl diphenyl phosphate (EHDPHP)>tris-2-butoxyethyl phosphate (TBOEP)>tris-2-chloroethyl phosphate (TCEP)>tris-1,3-dichloro-2-propyl phosphate (TDCIPP). The median sum PFR concentration for all 26 sites was 44ng/g lw (8.4ng/g ww), and levels ranged between 7.0 and 330ng/g lw (3.5 and 45ng/g ww). Levels and profiles of PFRs in eels showed that sampling locations and river basin catchments are possible drivers of spatial variation in the aquatic environment. Median PFR concentrations were lower than those of polybrominated diphenyl ethers (PBDEs) and hexabromocyclododecanes (HBCDs). No correlation was observed between the PFR concentrations and lipid contents, suggesting that the accumulation of PFRs is not primarily associated with lipids. Human exposure to PFRs, due to consumption of wild eels, seems to be of minor importance compared to other potential sources, such as inhalation and ingestion of indoor dust. Nevertheless, considering the very limited data available on PFRs in human dietary items and their expected increasing use after the phase out of PBDEs and HBCDs, further investigations on PFRs in biota and human food items are warranted.

Controlled reproduction inAnguilla anguilla(L.): comparison between spontaneous spawning and stripping-insemination approaches

This study aimed to compare the fertility of eggs between artificially matured female silver eels that spawned spontaneously and those that were spawned by manual stripping. The effects of the two methods of spawning on ovulation and fertilization rate were also investigated. For this purpose, 18 wild female European eels captured in Bonello lagoon (North Adriatic Sea) were carp pituitary extract-injected to undergo sexual maturation and ovulation; a final injection of 17,20β-dihydroxy-4-pregnen-3-one (DHP) was administered when at least 30% of the oocytes were fully transparent. After the DHP-injection, nine eels were transferred to a new closed recirculating aquaculture system, where they were housed with spermiating males (sex ratio 4/1) to allow spontaneous spawning (SPT-group); the remaining nine eels were transferred to a 250 L tank and ovulation was checked at four-hourly intervals by manual stripping (STR-group). The number of eggs per female in the SPT-group was significantly greater than that in the STR-group. Furthermore, fertilization rates in the SPT-group were notably higher than those observed in the STR-group. Significantly, the best performances were obtained among eels in which at least 50% of oocytes were fully transparent at the time DHP was administered. We conclude that the fertility of eggs from spontaneously spawning eels is superior to that of eggs acquired by strip-spawning and artificial fertilization.

Temperature modulates the progression of vitellogenesis in the European eel

article i nfo Wild female European eels were matured with CPE (carp pituitary extract) under three thermal regimes, two of which were variable (T10-15 and T15-18, moving from 10 to 15 °C and from 15 to 18 °C, respectively) and one constant, at 18 °C (T18). Before and during hormonal treatment, the eels were sampled and biometric measure- ments were taken. Immunoassays of sex steroids and vitellogenin were performed, as well as qPCR analyses of gene expression (ovarian cyp19a1) and ovarian histology. Prior to the hormonal treatment, the silver eels which had been maintained at 18 °C showed lower 11-KT and E2 plasma levels compared to those maintained at 10 °C. In addition, in the early vitellogenic stage, the androgen and cyp19a1 levels were lower at 18 °C than at 10 °C. Both these results and the positive correlations found between GSI and 11-KT (at the PV stage) and between oocyte diameter and cyp19a1 levels (in the EV stage) suggest that early ovarian development is facili- tated at low temperatures. Vitellogenesis was induced by CPE in all the thermal groups, but progression to the mid-vitellogenic stage was only observed after an accumulation of 900-1200 °D, at 15 or 18 °C, and progression to the late vitellogenic stage was only observed after an accumulation higher than 1300 °D, at 18 °C. Although temperature increased the rate of CPE-induced ovarian development, our results clearly indicate that this in- crease is not linear, but exponential, with acceleration in the increase of GSI at 18 °C from the mid-vitellogenic stage, or after an accumulation of 1300 °D. For the first time, a down-regulation of ovarian cyp19a1 caused by high temperatures in CPE-treated eels was observed. These results demonstrate that temperature can modulate eel ovarian development both before and after exogenous hormonal stimulations, and this knowledge could be used to manipulate the timing of vitellogenesis progression under laboratory conditions.

Controlled reproduction in the wild European eel (Anguilla anguilla): two populations compared

This study aimed to describe the response variability of female silver eels in terms of gonad development and eggs production to a standardized gonadotropic treatment (Carp pituitary extracts—CPE), and to relate this variability to population characteristics. For this purpose, sexual maturation, ovulation, and fertilization were induced in two eel populations coming from different locations in Adriatic Sea (Comacchio—CM and Marano-Grado—MG lagoons), and after that, their reproductive capacity was valuated. External (Silver index—SI, Eye index—EI, Pectoral fin length index—PFLI, Condition factor—K) and hormonal (17β-estradiol—E2, testosterone—T) parameters were measured, and some subject/group were killed for histological and lipid analysis and age determination. Morphometric parameters showed the CM-Group to have highest values of Body weight (BW), Body length (BL), and K, while MG-Group presented highest PFLI and Gonadosomatic index (GSI) values. Regarding hormonal analysis, the CM-Group showed significantly higher T and E2 levels than the MG-Group, both groups showed considerably rapid increase at T5 (5th injection). A positive trend in gonadal development was found through histological evaluation; a more regular maturation was observed in the MG-Group, whereas the CM-Group presented an exponential oocytes development starting from T10 (10th week), which led to an anticipated spawning. Lipid content showed significant differences in T0 (start study), post-ovulation, and Control (30th week) between CM and MG eels. As to zootechnical performances, while MG eels released spontaneously into the water, the CMs were stripped in order to check ovulation. The MG eels were statistically the most productive with 40.1 ± 6.33 % BW of eggs released. Furthermore, CM females ovulated mainly between the 19th and 22nd week (77.8 % spawned eels) instead in the MG’s ovulation goes from the 24th to the 28th week (100 % spawned eels). As fertilization is of concern, in both groups fertilized eggs were obtained with no difference in larvae production. These results seem to indicate that bigger dimensions, higher K, and larger lipid content (Comacchio eels) could fasten gonadic maturation without positively influencing reproductive performance of animals, both in term of quantity and quality of produced eggs. Smaller females with a highest SI (Marano-Grado eels) presented a more regular gonadic development, leading the animals to spontaneous spawning.

Modification of essential fatty acid composition in broodstock of cultured European eelAnguilla anguillaL

Farmed eels had lower levels of arachidonic acid (20:4 n-6) (ARA) and higher ratios of eicosapentaenoic acid (20:5 n-3) (EPA):ARA compared to wild European eels collected from the Baltic Sea and southern Norwegian coast. Eels fed a formulated feed (JD) with a distribution of essential fatty acids (EFA) resembling wild European eel were sampled after 0, 5, 10, 14 and 44 weeks of feeding to examine changes in fatty acid composition (FAC) in ovaries, visceral fat and muscle. The results showed a slow but steady incorporation of EFA. Lipids are incorporated in the oocytes early in oogenesis, and the leading cohort of oocytes is rich in lipid droplets before the onset of vitellogenesis. This indicates that feeding with optimized broodstock feeds should start early to allow the incorporation of EFA in the first cohort of oocytes. At least 14 weeks of feeding is required to change lipid EFA in broodstock eel to resemble EFA in the diet or in wild fish. After 44 weeks of feeding, ARA was significantly higher in the neutral lipids of ovaries (1.9%) compared to visceral fat (1.2%) or muscle (1.0%). EPA:ARA ratios decreased two- to threefold in all tissues examined during that time. ARA and docosahexaenoic acid (22:6 n-3) (DHA) had accumulated in ovarian polar lipids.

Induction of Sexual Maturation in Wild Female European Eels (Anguilla anguilla) in Darkness and Light

Sexual maturity was induced in wild silver eel females of the European eel (Anguilla anguilla) using a protocol similar to that successfully used for Anguilla japonica. Eels were injected for sixteen weeks with increasing doses of carp pituitary extract (10, 20, 30, and 40 mg CPE/kg body wt) and kept in natural light or completely dark conditions. In both groups, final oocyte maturation and ovulation were induced by injection of 17,20 h-dihydroxy-4-pregnen-3-one (DHP) at 2 mg/g body wt. All sixty eels turned out to be silver and actively migrant (58 in stage V and two in stage IV), showing a particularly high eye index (15.75±1.40), and distributed within three age classes (7-9 years). In the group kept in darkness, one of the four females ovulated 12 h after DHP injection in the thirteenth week. The remaining three females received the final (16th) CPE injection and ovulated 12-36 h after the DHP injection. In the group kept in light, two of the four females received the 16th CPE injection and ovulated 12-36 h after the DHP injection; the remaining two did not ovulate within 36 h post DHP injection. The total number of eggs released by the group kept in darkness (1,485,600) was significantly higher than that of the group kept in light (274,000). There was no mortality during the experiment.

&lt;i&gt;Aeromonas hydrophila&lt;/i&gt; subsp. &lt;i&gt;dhakensis&lt;/i&gt; Isolated from Feces, Water and Fish in Mediterranean Spain

Eight Aeromonas hydrophila-like arabinose-negative isolates from diverse sources (i.e., river freshwater, cooling-system water pond, diseased wild European eels, and human stools) sampled in Valencia (Spain) during 2004–2005, were characterized by 16S rRNA gene sequencing and extensive biochemical testing along with reference strains of most Aeromonas species. These isolates and all reference strains of A. hydrophila subsp. dhakensis and A. aquariorum showed a 16S rRNA sequence similarity of 99.8–100%, and they all shared an identical phenotype. This matched exactly with that of A. hydrophila subsp. dhakensis since all strains displayed positive responses to the Voges-Prokauer test and to the use of dl-lactate. This is the first report of A. hydrophila subsp. dhakensis recovered from environmental samples, and further, from its original isolation in India during 1993–1994. This was accurately identified and segregated from other clinical aeromonads (A. hydrophila subsp. hydrophila, A. caviae, A. veronii biovars veronii and sobria, A. trota, A. schubertii and A. jandaei) by using biochemical key tests. The API 20 E profile for all strains included in A. hydrophila subsp. dhakensis was 7047125. The prevalence of this species in Spanish sources was higher for water (9.4%) than for feces (6%) or eels (1.3%). Isolates recovered as pure cultures from diseased eels were moderately virulent (LD50 of 3.3×106 CFU fish−1) to challenged eels in experimental trials. They were all resistant to ticarcillin, amoxicillin-clavuranic acid, cefoxitin, and imipenem, regardless of its source. Our data point to A. hydrophila subsp. dhakensis as an emerging pathogen for humans and fish in temperate countries.

Movements of the European eel (Anguilla anguilla) in a chalk stream

Abstract – The movements of wild European eels (Anguilla anguilla L.) were monitored continuously over a 2‐year period in a tributary of the River Itchen, Hampshire, UK, using a passive integrated transponder (PIT) antenna system. The time of these movements was then related to a number of monitored and calculated environmental parameters. No eel movements were recorded in the winter months when mean stream temperature was below 10 °C. The numbers of movements peaked in May, when mean water temperatures and day length were increasing. The movement of eels was significantly (P &lt; 0.001, R2 = 0.45, N = 72) correlated with the time of sunset, with the majority (72%) recorded during the hours of darkness. The results of this investigation have implications for the design/timing of surveys of yellow eel populations and their habitat associations and suggest that successfully determining the microhabitat preferences of active resident eels will require remote monitoring throughout the 24‐h period.

Development and validation of a two-step real-time RT-PCR for the detection of eel virus European X in European eel, Anguilla anguilla

Eel virus European X (EVEX) is one of the most common pathogenic viruses in farmed and wild European eel (Anguilla anguilla) in the Netherlands. The virus causes a hemorrhagic disease resulting in increased mortality rates. Cell culture and antibody-based detection of EVEX are laborious and time consuming. Therefore, a two-step real-time reverse transcriptase (RT-)PCR assay was developed for rapid detection of EVEX. Primers and probe for the assay were designed based on a sequence of the RNA polymerase or L gene of EVEX. The real-time RT-PCR assay was validated both for use with SYBR Green chemistry and for use with a TaqMan probe. The assay is sensitive, specific, repeatable, efficient and has a high r2-value. The real-time RT-PCR assay was further evaluated by testing field samples of European eels from the Netherlands, which were positive or negative for EVEX by virus isolation followed by an indirect fluorescent antibody test. The real-time RT-PCR assay allows rapid, sensitive and specific laboratory detection of EVEX in RNA extracts from 10% eel organ suspensions and cell cultures with cytopathic effects, and is a valuable contribution to the diagnosis of viral diseases of eel.

Immunolocalization of steroidogenic enzymes in gonads of European eel,Anguilla anguilla(L.)

Summary The localization of cytochrome P450 cholesterol side-chain cleavage (P450scc), 3β-hydroxysteroid dehydrogenase (3β-HSD) and aromatase (P450arom) was investigated using polyclonal antibodies during gonad development in wild European eels, Anguilla anguilla (L.), from the River Po Delta (Ferrara, Italy). The first steroidogenic cells, observed in undifferentiated gonads of 14–16 cm yellow eels, showed no P450scc, 3β-HSD or P450arom activity, but positive regions appeared in head kidney insulae from this stage until the silver eel stage. In undifferentiated gonads of 16–20 cm yellow eels the steroidogenic cells were positive to all enzymes. Pre-Leydig steroidogenic cells, identified in Syrski organs of yellow eels of 22–26 cm evolving into testes, were positive to 3β-HSD and P450scc, but negative to P450arom. However, steroidogenic cells in Syrski organs evolving towards ovaries and in small but fully differentiated ovaries were positive to all enzymes. Immature testes of yellow and silver eels had Leydig cells positive to P450scc and 3β-HSD; the same reactions were also observed in some Sertoli cells of silver eel testes containing meiotic cells. Sex differentiation in A. anguilla apparently occurs through an initial female stage controlled by P450arom activity. Leydig and Sertoli cells appear involved in different steps of hormonal control of spermatogenesis: Leydig cells begin their steroidogenic activity before meiosis, while Sertoli cells begin their activity during meiosis.

Occurrence of Edwardsiella tarda in wild European eels Anguilla anguilla from Mediterranean Spain

Pure cultures of Edwarsiella tarda were isolated from body ulcers and internal organs of wild European eels caught in a Mediterranean freshwater coastal lagoon (Albufera Lake, Valencia, Spain) over a 1 yr period. Overall, the E. tarda isolation rate from wild eels was 9%, but this increased to 22.8% in diseased individuals. All 22 E. tarda isolates belonged to the 'wild-type' biogroup of the species and were virulent for eels (lethal dose that kills 50% of exposed individuals [LD50 dose]: 10(4.85) to 10(6.83) CFU ind.(-1)), and therefore represented the aetiological agent of the haemorrhagic disease observed in wild European eels. The E. tarda isolates and E. tarda CECT 894T type strain were biochemically and serologically related and resistant to macrolides, antifolates, and glycopeptides, but only the isolates from wild eels were resistant to clindamicyn. This study is the first description of edwardsiellosis in a wild European eel population, and alerts us to the presence of E. tarda in natural wetland environments in Mediterranean Europe.

First description of nonmotileVibrio vulnificusstrains virulent for eels

Nonmotile Vibrio vulnificus strains were isolated as pure cultures from body ulcers and internal organs of wild diseased European eels caught in a Mediterranean freshwater coastal lagoon. All 54 V. vulnificus isolates were nonmotile, indole-, ornithine decarboxilase-, mannitol- and cellobiose-positive, developed the opaque variant in culture, belonged to the O-antigenic serovar A and were highly virulent for eels by both intraperitoneal injection and immersion challenges. The nonmotile phenotype found in our V. vulnificus isolates was stable: nonmotile cells were always recovered from experimentally infected eels; no variation in the immobility of the V. vulnificus cells was observed for repeated subculture by daily passages on solid media, at different temperatures or incubation times and with or without magnesium sulfate. Many of the fla genes of Vibrio were present in the genome of the nonmotile strains (flaCDE and flaFBA for flagellins and flaH for the distal capping protein), although we observed by transmission electron microscopy that these V. vulnificus strains always lacked the polar flagellum. This is the first report on the existence of nonmotile wild-type V. vulnificus strains.

Predicting catches for the Lough Neagh (Northern Ireland) eel fishery based on stock inputs, effort and environmental variables

Abstract The Lough Neagh eel fishery is the largest remaining commercial source of wild European eels. Following the pan‐European elver crash in the 1980s, the numbers of elvers entering the Lough fell drastically, prompting the Lough Neagh Fishermen's Co‐operative Society Ltd (LNFCS) to purchase additional glass eels from a UK distributor. LNFCS have continued to purchase glass eels, when market prices have been favourable, to supplement the natural glass eel input. This study assesses the impact of these stocked glass eels on the Lough Neagh yellow and silver eel fisheries with a predictive model based on natural and stocked glass eel input, effort and environmental variables. Despite the final models’ limitations to assess conclusively the contribution of the additional purchased glass eels the models can be used cautiously to predict, in the short term, yellow and silver eel output. The models will require reviewing, on an annual basis, as further data become available.